Conventionally, a parameter called Time To Collision (TTC) has been used as an index for a degree of risk of collision of a subject vehicle with an obstacle as described in Japanese patent document JP-A-2005-324700 (This document is also published in United States as 2005-0267683). The parameter TTC represents a period of time to a collision of the subject vehicle to the obstacle.
The TTC represents, in other words, the period of time to the collision if the subject vehicle maintains a traveling condition at the time of the calculation of the TTC. Further, by another definition, the TTC is a “100% probability” of the collision of the subject vehicle if the subject vehicle maintains a “current” traveling condition at the time of TTC calculation.
The TTC is effective for representing a degree of risk of collision of the subject vehicle with the obstacle that exists in front of the subject vehicle (a front object) when the vehicle is traveling in a straight path. However, when the traveling condition of the subject vehicle is under an effect of a sideway acceleration, the TTC does not serve as a correct index for representing the degree of risk of collision with the front object.
For example, when a driver of the subject vehicle operates a steering wheel to avoid the collision with the front object, the TTC does represent the correct index of the degree of risk of the collision with the front object during the period of time before the driver starts operating the steering wheel (i.e., while the subject vehicle is traveling in the straight path).
Once the steering wheel operation has started for avoiding the collision, the traveling condition of the subject vehicle transits from a straight travel condition to a sideway acceleration affected condition. In the sideway acceleration affected condition, the TTC is calculated based on the distance of the subject vehicle from the front object, and a relative speed relative to the front object. That is, the TTC is calculated based on the variables that are defined only by the motion of the subject vehicle in a front-rear direction, and not on the variables defined by the motion in the lateral direction.
Therefore, if the traveling condition of the subject vehicle generates the sideway acceleration of the subject vehicle, the TTC does not represent the correct index of the degree of risk of the collision of the subject vehicle with the front object, thereby yielding a false value of the degree of danger (i.e., 100% probability) of the collision of the subject vehicle with the front object even after the subject vehicle has started the collision avoidance motion.